linux-kernel - RE: [PATCH v2 08/59] x86/build: Ensure proper function alignment

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Date:   Mon, 5 Sep 2022 02:09:02 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "Josh Poimboeuf" <jpoimboe@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        Joao Moreira <joao.moreira@...el.com>,
        Joseph Nuzman <joseph.nuzman@...el.com>,
        "Steven Rostedt" <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        "Masami Hiramatsu" <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: RE: [PATCH v2 08/59] x86/build: Ensure proper function alignment

From: Peter Zijlstra
> Sent: 02 September 2022 14:07
> 
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> The Intel Architectures Optimization Reference Manual explains that
> functions should be aligned at 16 bytes because for a lot of (Intel)
> uarchs the I-fetch width is 16 bytes. The AMD Software Optimization
> Guide (for recent chips) mentions a 32 byte I-fetch window but a 16
> byte decode window.
> 
> Follow this advice and align functions to 16 bytes to optimize
> instruction delivery to decode and reduce front-end bottlenecks.

Performance figures?

IIRC the same document will suggest aligning all jump labels.
That is pretty much known to be harmful because of the bloat
it generates.

Also things like CFI and ENDBRA have a habit of making the
entry point unaligned unless you can pad to 16n+x values.

	David

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